Convolvulus Sagittatus Root

Acyl residue: Bonabilic acid

(rel. configuration)

Acyl residue: Bonabilic acid

(rel. configuration)

Acyl residue: Apobonabilic acid

(rel. configuration)

Acyl residue: Apobonabilic acid

(rel. configuration)

Fig. 3.22 Bonabilines, unique alkaloids from the roots of Bonamia spectabilis (Convolvulaceae); highlighted in grey: structurally related components of certain (non-convolvulaceous) essential oils for comparison. a-Pinene, myrtenol, and myrtenal are constituents of the essential oil of Myrtus communis L., Myrtaceae (Savikin-Fodulovic et al. 2000); the corresponding carboxylic acid, myrtenic acid, was detected as a constituent of the essential oil of Cedronella canariensis (L.) Webb. & Berthel., Lamiaceae (Engel et al. 1995)

Fig. 3.22 Bonabilines, unique alkaloids from the roots of Bonamia spectabilis (Convolvulaceae); highlighted in grey: structurally related components of certain (non-convolvulaceous) essential oils for comparison. a-Pinene, myrtenol, and myrtenal are constituents of the essential oil of Myrtus communis L., Myrtaceae (Savikin-Fodulovic et al. 2000); the corresponding carboxylic acid, myrtenic acid, was detected as a constituent of the essential oil of Cedronella canariensis (L.) Webb. & Berthel., Lamiaceae (Engel et al. 1995)

identified in Dichondra sericea, Falkia repens (Dichondreae), Bonamia semidigyna (Cresseae), Maripa panamensis (Maripeae), Convolvulus chilensis, C. dorycnium, C. farinosus, C. glandulosus, C. graminetinus, C. sagittatus, C. siculus (Convolvuleae), and Ipomoea plebeia (Ipomoeae). It is remarkable that there was only one species T5-positive in this latter large tribe and not even any species in the "Merremieae". Modified, rearranged phenylpropanoids like tropic acid or phe-nyllactic acid are confined to the Solanaceae.

3.4.3.3 Rare Esters of 3a-Hydroxytropane/-Ā«ortropane (T6)

Bonabilines (T6-A). Besides rather common tropanes (T2, T4) two unique congeners, bonabiline A and B, were discovered in the roots of Bonamia spectabilis, a climbing shrub of tropical Africa including Madagascar (Ott et al. 2006). The acyl residues of the bonabilines are provided by monoterpenoic acids (Fig. 3.22), a novel combination of ester alkaloids of any type. The free acids, bonabilic acid and its

Merresectine E Consiculine

4'-Dihydroconsabatine

Fig. 3.23 Proposed biogenetic relationships between the prenylated aromatic 3a-acyloxytropane merresectine E and congeners confined to Convolvulus sabatius, C. siculus, and Merremia quinata (Convolvulaceae) whose structure has lost the aromatic character of the acyl moiety

Consabatine

4'-Dihydroconsabatine

Fig. 3.23 Proposed biogenetic relationships between the prenylated aromatic 3a-acyloxytropane merresectine E and congeners confined to Convolvulus sabatius, C. siculus, and Merremia quinata (Convolvulaceae) whose structure has lost the aromatic character of the acyl moiety dehydration product apobonabilic acid, were also unknown before. The latter represents an oxidation product of the bicyclic pinane-type monoterpene verbenol, a constituent of certain essential oils, e.g., the one of olibanum from the bark of Boswellia Roxb. ex Colebr. spp., Burseraceae (Guenther 1943). Such cyclic monoterpenoic acids in general seem to be very rare in the plant kingdom; myrtenic acid has been reported as trace compound in certain essential oils (Engel et al. 1995).

Consabatines, Consiculine (T6-B). Two tropane alkaloids with another unique type of acyl moiety were discovered in the roots of two Convolvulus species of Mediterranean origin, C. siculus, a small herbaceous twiner, and C. sabatius ssp. mauritanicus, a suffrutex used as a popular ornamental (Jenett-Siems et al. 1998b). These metabolites, consiculine and consabatine, are characterized by acyl residues, provided by consiculic acid and 2-deoxyconsiculic acid, respectively, which showed structural similarities to the one of merresectine E (Fig. 3.23). They share

(i) the prenylated six-membered ring, (ii) an oxygen function in the ortho-position of the prenyl residue, and (iii) the carboxyl in the meta-position of this prenyl. However, they differ in that (i) the six-membered ring represents a cyclohexene moiety instead of an aromatic one and (ii) there is a hydroxy substituent geminal to the carboxyl. Indeed, merresectine E and other merresectines occur together with consiculine/consabatine in the roots of C. sabatius and in the epigeal vegetative parts of M. quinata (Jenett-Siems et al. 2005b) suggesting a biogenetic relationship. It may be concluded that merresectine E has to be the precursor of consabatine/ consiculine rather than vice versa. Otherwise it would not be plausible that merresectines are present in, e.g., seven Merremia spp. which did not even show traces of consabatine/consiculine. In conclusion the ring of the acyl residue of merresectine E seems to loose its aromatic character in certain convolvulaceous species. Such

3a-Nicotinoyloxytropane Ipomoea argillicola / Merremia vitifolia

Merredissine Merremia dissecta

3a-Nicotinoyloxytropane Ipomoea argillicola / Merremia vitifolia

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